Fluidization is typically observed when a dense phase bed of solid particles comes in contact with a vertical upward fluid flow, in an intermediate range of flow rates. At low flow rates or velocities, the solid particles lie on one another in a dense phase state. At high flow rates or velocities, the solid particles are conveyed upward and out of a vertical column. This is known as hydraulic or pneumatic transport. At intermediate flow or velocity ranges, each particle becomes individually suspended in the fluid as it flows past the particles. On the whole, the bed of solid particles remains motionless relative to the walls of the vertical column. This bed is considered to be fluidized.
A typical system for fluidizing solid particles uses a cylindrical vertical column. Prior to fluidization, a surface of the dense phase bed of solid rests against a fluid permeable support which is affixed to the vertical column. Fluidization is initiated by flowing either a liquid or vapor fluid into the vertical column below the fluid permeable dense phase bed support. The permeable dense phase bed support acts as a distributor as the flowing fluid passes through the support, and the fluid is directed to evenly contact the supported surface of the dense phase bed. After contacting the supported dense phase bed surface, the fluid flows through the entire dense phase bed where the fluid comes into contact with the upper surface of the bed. As the fluid contacts the upper surface, fluidization of the dense phase bed begins. In essence, fluidization in such a column is the result of the flowing fluid pushing the solid particles away from the dense phase portion of the bed. Fluidization itself is actually initiated only after the flowing fluid has passed through the entire dense phase bed.
Various commercial fluidization systems are described by Squires et al., "Fluidized Beds: At Last, Challenging Two Entrenched Practices", Science, 1985, 230, 1329-1337. Squires demonstrates that the typical fluidization systems follow either the "German" practice for treating course solids or the "American" practice for dealing with fine powders.
A problem with the typical dense phase system is that the fluid which initially contacts the dense phase bed must flow completely through the dense phase bed before fluidization begins. This type of system requires a rather lengthy amount of time to reach steady state conditions in which an entire dense phase bed of solids becomes continuously fluidized. Such a system is not desirable, for example, when the solids are catalyst particles, and it is desirable to quickly reach fluidized steady state reaction conditions.